1. Technical Field
The present invention relates to a booster for use in a brake mechanism of an automobile, in particular, an electric booster that utilizes an electric actuator as a boost source.
2. Prior Art
Conventionally, vacuum brake boosters are commonly used in automobile brake mechanisms. Such vacuum brake boosters utilize a negative pressure in an engine intake pipe to boost an input to generate an increased output. In recent years, however, with the development of engines having an improved fuel efficiency, cleaner exhaust gas, and so on, intake pipes have tended to be designed so as to have a relatively low negative pressure. In order to ensure a desired boosting performance or responsiveness of such a vacuum brake booster, a measure must be taken, for example, to increase a size of the vacuum brake booster, to increase a negative pressure by using an ejector, or to provide the vacuum brake booster with an engine-driven vacuum pump. Use of such measures, however, makes it more difficult to mount the vacuum brake booster in a vehicle, and increases costs.
For these reasons, attention has been directed to electrically actuated brake boosters that utilize an electric actuator as a boost source. Such electrically actuated brake boosters are disclosed, for example, in patent references 1 and 2 (Japanese Patent Public Disclosures No. HEI 10-138909 and HEI 10-138910). These electrically actuated brake boosters comprise: a main piston (a shaft member) adapted to move forward and backward in response to operation of a brake pedal; a booster piston (a tubular member) that is fitted around the main piston and is movable relative to the main piston; and an electric actuator for moving the booster piston forward and backward. The main piston and the booster piston function as pistons of a master cylinder, the front ends of the main piston and the booster piston defining a pressure chamber of the master cylinder, such that a hydraulic brake pressure is generated in the master cylinder by an input thrust transmitted to the main piston from the brake pedal and a booster thrust transmitted to the booster piston from the electric actuator.
Therefore, in the electrically actuated boosters of the above-mentioned references 1 and 2, a part of a reaction force resulting from a hydraulic brake pressure generated in the pressure chamber of the master cylinder acts on the main piston (shaft member), while another part of the reaction force acts on the booster piston (tubular member). Thus, when an attempt is made to increase a thrust of the booster piston and thereby increase a hydraulic brake pressure in the master cylinder, for activating a brake assist, the hydraulic brake pressure acts as a reaction force on the main piston so as to move back the main piston in coordination with the brake pedal.
In other words, to obtain a desired braking force, an input thrust (a pedal-stepping force) needs to be increased to prevent the main piston from moving backward. As a result, a boost ratio for the input thrust fails to increase.
The present invention was made in view of the above-mentioned problem associated with the conventional brake boosters. An object of the present invention is to provide an electrically actuated booster that is capable of obtaining a desired boost ratio for a given input thrust when an electric actuator is activated as a boost source, to thereby improve brake pedal operability.